1. Field of the Invention
The present invention relates generally to an ultrasonic system for detecting the presence of air in a fluid line, and more particularly to a system using a windowing technique to determine when a threshold amount of air bubbles in a predetermined volume of material pumped occurs, with the alarm being sounded only when this threshold amount of air bubbles is present in the window volume, thus avoiding nuisance alarms.
In the past there have been two primary techniques which have been used to deliver drugs which may not be orally ingested to a patient. The first such technique is through an injection, or shot, using a syringe and needle which delivers a large dosage at relatively infrequent intervals to the patient. This technique is not always satisfactory, particularly when the drug being administered is potentially lethal, has negative side effects when delivered in a large dosage, or must be delivered more or less continuously to achieve the desired therapeutic effect. This problem results in smaller injections being given at more frequent intervals, a compromise approach not yielding satisfactory results.
Alternatively, the second technique involves administering a continuous flow of medication to the patient, typically through an IV bottle. Medication may also be delivered through an IV system with an injection being made into a complex maze of IV tubes, hoses, and other paraphernalia. With drop counters being used to meter the amount of bulk fluid delivered, many medications still end up being administered in a large dosage through an injection into the IV lines, although the medications may be diluted somewhat by the bulk fluid.
As an alternative to these two techniques of administering medication to a patient, the relatively recent addition of medication infusion pumps has come as a welcome improvement. Medication infusion pumps are utilized to administer drugs to a patient in small, metered doses at frequent intervals or, alternatively, in the case of some devices, at a low but essentially continuous rate. Infusion pump therapy may be electronically controlled to deliver precise, metered doses at exactly determined intervals, thereby providing a beneficial gradual infusion of medication to the patient. In this manner, the infusion pump is able to mimic the natural process whereby chemical balances are maintained more precisely by operating on a continuous time basis.
One of the requirements of a medication infusion system is dictated by the important design consideration of disposability. Since the portion of the device through which medication is pumped must be sterile, in most applications of modern medication infusion equipment some portions of the equipment are used only once and then disposed of, typically at regular intervals such as once daily. It is therefore desirable that the fluid pump portion of the infusion pump device be disposable, with the fluid pump being designed as an attachable cassette which is of inexpensive design, and which is easily installable onto the main pump unit.
It will be perceived that it is desirable to have a simple disposable cassette design to minimize the cost of construction of the cassette, using the minimum number of parts necessary in the design of the cassette. The design of the cassette must be mass producible, and yet result in a uniform cassette which is capable of delivering liquid medication or other therapeutic fluids with a high degree of accuracy. The cassette should include therein more than just a fluid pump; other features which have formerly been included in peripheral devices may be included in the cassette.
Such a system has been disclosed in all of the above-identified previously filed related applications. Of these applications, U.S. Ser. No. 128,121, entitled "Air-In-Line Detector for a Medication Infusion System," is hereby incorporated herein by reference.
An essential function of a medication infusion system is to avoid the infusion of fluid containing more than a minimal amount of air bubbles therein. Although steps may be taken to minimize the possiblility of air bubbles being contained in a fluid which is to be infused to a patient, it is essential to monitor the fluid line before it reaches the patient to ensure that air bubbles remain in the fluid which is to be infused are detected. The detection of air bubbles in all fluids which are to be infused is therefore a critical design requirement.
One type of air-in-line detector which has been used in the past is an ultrasonic detector, which places an ultrasonic transmitter on one side of a fluid line and an ultrasonic receiver on the other side of the fluid line. Fluid is a good conductor of ultrasonic energy while air or foam is not. Accordingly, if there is an air bubble in the fluid line between the transmitter and the receiver, the signal strength will be greatly attenuated, and the presence of the bubble will be indicated. Examples of ultrasonic air-in-line detectors include U.S. Pat. No. 4,764,166, to Spani, and U.S. Pat. No. 4,821,558, to Pastrone et al.
Such ultrasonic air-in-line detectors work very well in detecting air bubbles in the fluid line. In fact, fairly small air bubbles can be detected with an ultrasonic detector. The problem with such ultrasonic detectors is that they are too good at detecting air bubbles. In fact, many medications are not degassed and contain some amount of air therein, which air may form small bubbles, particularly during the pumping process as performed in a medication infusion pump.
In most patients it is not harmful to pump such small amounts of air intravenously. Only a few patients can tolerate no air introduced into their venous systems, such as neonates, pediatrics, and those patients having septal defects. Other than when infusing fluid into such patients, or performing an intra-arterial infusion, the introduction of a very small quantities of air is not believed to be particularly harmful. In such cases where even small bubbles may be harmful, air eliminating filters may be used.
Thus, the situation presented is that the ultrasonic air-in-line detection system will alarm for any amount of air in the fluid line, irrespective of whether that amount is insignificant or large. Typically, hospital personnel tend to see such alarms for tiny amounts of air as nuisance alarms, since they tend to occur frequently due to the virtual inevitability of small bubbles being present. In fact, if a high number of nuisance alarms occurs, the device is likely to be seen as less desirable than a competing device with a poorer air-in-line detection system which does not alarm as often.
It is therefore the primary objective of the present invention to provide an air-in-line detection system which will not alarm merely due to the presence of a tiny amount of air detected in the fluid path. The system of the present invention must be of a design retaining all of the advantages of known ultrasonic air-in-line detection devices, and must also provide a number of additional advantages and improvements. Specifically, the air-in-line detection system of the present invention must be capable of detecting even very small air bubbles in the fluid line of a disposable cassette near the output end of the cassette, after the pumping operation has been performed. Then, the system of the present invention must discriminate between a situation in which isolated tiny air bubbles are present and the situation when a significant amount of air is present in the fluid line, not alarming in the former case and always alarming in the latter case.
Several other additional features are desirable in the design of a cassette and a main pump unit making up an air-in-line detection system. Examples of such features are the ability to detect air bubbles whether the flow rate of the fluid in the cassette is fast or slow, and the ability to detect air in the fluid line even when the interior of the fluid line remains coated with fluid. The system must be capable of accurately and effectively detecting air bubbles in any type of fluid which may be infused, whether the fluid is clear or opaque, as in the case of lipid solutions.
The system must accomplish all these objects in a manner which will retain and enhance all of the advantages of reliability, durability, and safety of operation. The system of the present invention must provide all of these advantages and overcome the limitations of the background art without incurring any relative disadvantage. All the advantages of the present invention will result in a superior medication infusion system having a number of advantages making the system a highly desirable alternative to systems presently available.